EPRI collaborated with Constellation Energy Nuclear Group in a field project at the Ginna nuclear plant to evaluate two new approaches for assessing the condition of concrete containment structures.

Constellation Energy Nuclear Group (CENG), EPRI, and the Department of Energy are evaluating the feasibility of tendon load monitoring and surface strain monitoring to assess the condition of concrete containment structures. These technologies are potentially more robust and cost-effective than current approaches, and can provide more detailed information about equipment condition to inform decision making and support aging management programs. A pilot assessment at the Ginna nuclear plant in New York demonstrated the feasibility of these concepts and identified actions to guide further technology development.

Like many pressurized water reactors, the Ginna containment includes tendons that can be tensioned to add strength and withstand a potential pressure increase from a breach in the primary system. To ensure the tendons are functioning properly, nuclear plant owners perform periodic "lift-off" tests to determine the force required to lift the head plate at the top of the tendons. If the force required to lift the tendon head plate is lower than a pre-determined level, the tendons are not tensioned properly and adjustments are made using shims.

As an alternative, the CENG project team installed fiber-optic strain gages on the main shims that support the head plates (under the protective orange covering in the photo). The gages feed real-time data on tendon load, strain and temperature to a computer, providing a wealth of information unobtainable from periodic lift-off testing. The fiber optic system gives plant engineers baseline information on tendon condition and allows them to track changes over time. Moreover, the system enables tracking and quantification of strain variation due to seasonal or diurnal temperature cycles or other factors. For example, plant engineers conducted a structural integrity test in which they pressurized the containment to simulate a loss of coolant accident, and the fiber optic system measured the increased strain on the tendons.

In a separate first-of-a-kind application, EPRI evaluated the performance of a digital image correlation (DIC) system at Ginna to monitor surface strain on the containment concrete outer surface for enhanced condition assessment. DIC technology uses twin high-resolution cameras to take photographs of a speckled pattern applied to a surface. A software system analyzes the camera images pixel by pixel and develops a map of the surface, providing a baseline measurement. Any subsequent stress or movement of the surface alters the speckled pattern and is detected by the cameras and analyzed by the software. The project team applied the DIC system during the containment pressurization test at Ginna and demonstrated its ability to detect and measure the strain on the concrete surface as well as changes in surface dimensions.

For more information, contact Rich Tilley at 704.595.2597 or rtilley@epri.com.